Pneumatic sound reproducer



I July 21, 1936. R. F, MALLlNA 2,048,508

PNEUMATIC SOUND REPRODUCER Filed May 5l, 1934 2 Sheets-Sheet 1 /NVENTORBy* R/-TMALL/NA A TTORA/EV July 21, 1936.'

PNEUMATI C S OUND REPRODUCER R. F. MALLINA 2,048,508

Filed May 51' 1954 2 sheets-sneer 2 I l y I l /N/ENTOR REMALL//VAATTORNEY Patented July 21, 1936 UNITED STATES Bell TelephoneLaboratories,

Incorporated,

New York, N. Y., a corporation of New York Application May 31, 1934,Serial No. 728,201 e claims. (ci. 179-113) This invention relates toimprovements in sound reproducing devices and particularly toloudspeakers of the pneumatic type in which currents representing thesounds to be reproduced operate 8 suitable means for modulating an airstream to produce large volumes of sound.

The object of the invention is an efficient and inexpensive modulatorfor loud-speakers of this type.

v According to one important feature of the invention, the modulator isheld in static balance by the action of the air stream therebyeliminating the turning movement of the air pressure on the modulator.

1'5 The preferred structure, according to the invention, comprises arubber damped balanced armature mounted for vibration about its medialaxis which is disposed transversely in the center of the air conduit toform two orifices equal in area between the 'tips of the armature andthe walls of the conduit. The armature extends through signal currentcoils -which are surrounded by bifurcated pole-pieces suitably contouredin the direction of the air ow and disposed to form working gaps withthe armature. Flux is fed to the pole-pieces by permanent orelectromagnets external to the conduit vand the vibrations of thearmature produced by the interaction of the steady and alternatingfluxes are damped by rubber blocks engaging both sides of the armaturenear the armature tips.

The armature may be mounted either at right angles to the axis of theconduit or at a suitable angle (such as 30 to 45) to the transverse axisof the conduit. The right angle construction is inherently of higherefciency but requires that the inner wall of the conduit be contoured toform the necessary oriiices. With the angular arrangement the efiiciencyis somewhat decreased but the axes of the oriiices may be parallel tothe axis of the conduit.

In the drawings. Fig. 1 is a sectional view of a modulator mounted atright angles to the axis of the conduit;

Fig. 2 is a sectional view taken as indicated in Fis- 1; l

Figs. 3 and 4 are detail views of the armature damping structure; 4

Fig. 5 is a sectional view of the alternate construction in `which theamature is mounted at an angle to the transverse axis of the conduit,Aand Fig. 6 is an assembly view showing the housing 55 for the oriceadjusting members of Fig. 1 and an arrangement of the magnets which-isapplicable to either embodiment of the invention.

In Fig. 1, the conduit Il composed of -brass or other non-magneticmaterial is connected at lone end to a source of compressed air by meansof 5 the pipe I2 and at the 'other end to a suitable horn of which onlya portion of the throat I3 is shown. The pole-'pieces I4, I5 extendacross the conduit and project through the walls on either side to4 makecontact with the magnets I6, I1 which are 10 shown in Fig. 6 aspermanent magnets but for loud-speakers of great output capacity thelarge magneto-motive force required may be supplied by electro-magnets.The signal current coils I8,

I9 are housed between the pole-pieces and the 15 armature 20 is pivotedat its center between the coils on the shaft 2I which is journaled inthe pole-piece spacing member 28 as shown in Fig. 2. The pole faces arerecessed to receive the rubber damping members 22, 23 which surround the20 armature as shown more clearly in Figs. 3 and 4 and hold it in itsproper position at right angles to the axis of the conduit. At thesection where the armature is pivoted, the conduit is substantiallyrectangular in cross-section and has slots 25 29, 30 extending the fullWidth o1' ythe armature. The slots are closed by housings 3|, 32 in'which the sliding blocks 33, 34 are fitted to form the orifices 24 and25. By means of the screws 35, v 36 the blocks may be accuratelyadjusted to make 3o the oriiices equal and ot any desired size. Thisadjustment is preferably made with the air applied to the modulator sothat the slight 'deflection of the armature due to the air pressure willnot result in orifices of unequal areas. 35

When the coils I8 and I9 are not carrying currents, the armature 20 isstationary in its midposition as shown and a continuous stream of airilows through the orifices to the horn but when currents representingthe sounds to be repro- 40 duced are supplied to the terminals 26, 21 ofthe coils, an 'alternating flux is set up longitudinally in the armatureand the armature vibrates between the pole faces in the well knownmanner. The vibrating armature tips vary the areas of the 45 orifices toproduce variations in the air stream corresponding to the signal currentvariations which are accordingly reproduced as audible sounds from thehorn.

Since the armature is rigidly pivoted at its center. the areas exposedto the air pressure on opposite sides of the axis are equal and thesteadyl'air pressure therefore has no tendency to displace the armaturefrom its center or neutral position. It is well known that pneumaticloud-speakers `Lili are capable of producing* sounds oi great volumesince a large proportion of the acoustic energy is derived from thecompressed air. Modulators which are not statically balanced, however,must be extremely rugged to withstand the unbalanced variessubstantially directly with the armature displacement.

Thevalternate form shown in Fig. 5 is, in general, similar inconstruction and operation to the structure just described. 'Iheessential difference is that the armature 40 is mounted at an angle offrom 30 to 45 to the transverse axis of the conduit so that axes of theorifices are parallelto the walls of the conduit 4I The configuration ofthe pole-pieces 42, 43, the coils 44, 45 and the damping members 46, 41is modified to adapt them to the inclined position o! the armature butobviously their functions are the same as inthe structure of Fig. 1.While the area of the orifices 48, 49 does not vary directly with thearmature displacement but rather `asthe cosine of the angle ofdisplacement wlth a consequent decrease in percentage modulation ascompared with the structure-of Fig. 1, this disadvantage may be morethan compensated by the advantages of a straight path on the flow of theair through the orifices.

What is claimed is: y

ll. In an acoustic device, a conduit for a fluid stream, a valvedisposed midway said conduit, said valve being mounted for rotationabout its medial axis transversely of the conduit so that it isstatically balanced in said conduit and electromagnetic meansincluding-pole pieces within the conduit defining streamlined passagesadjacent the valve for moving said valve in accordance with the sound tobe reproduced.

2. In an acoustic device, the combination with a conduit for a fluidstream and a modulator pivoted on its medial axis mounted in the conduitwith its axis disposed transversely of the conduit, of means forariiusting the area of the uid path around the ends of the modulator,and means for vibrating the modulator in accordance with the sounds tobe reproduced. Y 6

3. In an acoustic device, the combination with a conduit for a fluidstream, a pair o! furcated polepieces within the conduit and a pivotedmodulating member disposed between the pole-pieces and extending towardopposite walls ot the conduit, of l0 means for vibrating the memberbetween the polepieces in accordance with the sound to be reproduced,and means for independently adjusting the oriiices between the conduitwalls and the modulating member. f,

4. In an acoustic device, the combination with a conduit for a fluidstream, a pair oi' furcated pole-pieces within the conduit and anarmature pivoted on its medial axis in static balance between thepole-pieces and having end portions 2.0 v

forming 4I.equal oriflces adjacent the walls oi.' the conduit,of meansfor vibrating the armature in accordance with the sounds to bereproduced, and means for damping the armature vibrations. 5. In anacoustic device, the combination with a conduit for a uidstream, a pairo f furcated pole-pieces dividing the conduit into two passages adjacentthe walls ot the conduit, an armature pivoted on its medial axis andextending into said 30 passages, means for resiliently holdingv thearmature between the pole-pieces at an angle to the transverse axis ofthe conduit, and means for vibrating the amature in accordance with thesounds to be reproduced. 6. In an acoustic device, a conduit for a iuidstream, and an electromagnetic structure within the conduit comprisingpole pieces dciiningl air gaps and dividing the conduit into tworestricted passages, and a vibrating armature pivoted bef tween the polepieces projecting through the gaps into the passages and varying theilow oi' iiuid through the passages in accordance with its ownvibrations.

RUDOLPH F. MALLINA. 45

